STAGING: Declare request_cache as static for drivers/staging/lustre/lustre/ptlrpc...

Declare request_cache variable as static. It is only used by
drivers/staging/lustre/lustre/ptlrpc/client.c, and its naming is common
which will lead to namespace pollution.

Signed-off-by: Simon Guo <wei.guo.simon@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>